1. Field of the Invention
This invention relates to a two way clutch engageable and disengageable in forward and reverse directions.
2. Related Background Art
A one-way clutch is a mechanical part comprising a wedge member such as a cylindrical roller or a sprag provided between an inner race and an outer race rotatable coaxially with each other so as to transmit a torque only in one direction. A two-way clutch is endowed with the function of said one-way clutch in forward and reverse rotational directions. The two-way clutch is applied to the automatic changeover or the like of a four-wheel drive/two-wheel drive, and the principle thereof will hereinafter be described.
FIG. 1A and 1B of the accompanying drawings illustrates the changeover mechanism of four-wheel drive/two-wheel drive utilizing the two-way clutch. FIG. 1A shows the state of the two-way clutch when the engaged vehicle is moving forward. An outer race 20 is provided with a roller receiving portion 21 comprised of bisymmetrical arch-shaped cam surfaccs 21a and 21b, and a cylindrical roller 30 which is a wedge member is inserted in the roller receiving portion 21 and between the cam surfaces and an inner race 10. The reference numeral 40 designates a cage for supporting the cylindrical roller 30. For purpose of the prsent description, it is to be understood that the vehicle is front-wheel-driven during two-wheel drive and the outer race 20 of the two-way clutch is connected to a power source, such as an engine generating a drive force, and the inner race 10 is connected to rear wheels which are driven (non-driving) wheels. When the vehicle is running by two wheel drive (i.e., front wheel drive), the outer race 20 of the clutch is connected to the power source such as the engine and rotates in the direction of arrow A, while the rear wheels freely rotate only by the frictional force thereof with the ground, which rotation is transmitted to the inner race 10, which is thus rotated in the same direction as the outer race (the direction of arrow A). Under normal conditions, the extraneous gear ratio or the like is suitably selected so that the rotational speed of the inner race may always be higher by several percent than the rotational speed of the outer race 20 which transmits the drive force. Since as described above, the rotation of the inner race is normally faster than the rotation of the outer race, the roller 30 is subjected to a frictional force in the leftward direction as viewed in the figure, and the cam surface 21b and the roller 30 are in their free state, so that the inner and outer races rotate freely relative to each other. When the front wheels which are the driving wheels slip for some reason, the running speed of the vehicle drops sharply and the rotational speed of the inner race 10 connected to the rear wheels which are the driven wheels begins to drop. When the slipping condition of the front wheels continues and the speeds of the inner and outer races become equal to each other, the roller 30 loses the a leftwardly acting frictional force created from the difference between the speeds of the inner and outer races and wedges between the cam surface 21b and the inner race 10, whereby the clutch becomes engaged. When such a state is brought about, the torque of the outer race 20 is transmitted to the inner race 10 and the drive force is also transmitted to the rear wheels, thus bringing about a four-wheel drive state. When in the four-wheel drive state, the vehicle gets out of its slipping condition with the aid of the drive force of the rear wheels, the vehicle begins to move normally again and the rotation of the rear wheels increases the number of rotations of the inner race of the clutch. When the rotational speed of the inner race exceeds the rotational speed of the outer race, the roller loses its wedge action again and the clutch becomes free, and the ordinary twowheel drive condition is restored.
FIG. 1B shows the engaged state of the aforedescribed clutch mechanism during the backward movement of the vehicle. The inner and outer races rotate in the direction of arrow B which is opposite to the rotational direction thereof during the forward movement of the vehicle. During the backward movement, as shown, the position of the cage 40 is shifted to left as viewed in the figure, and the roller 30 cooperates with the cam surface 21a to effect a wedge action. Since as described above, the rotational speed of the inner race of the clutch connected to the rear wheels of the vehicle normally higher than the rotational speed of the outer race, the clutch rotates freely during the normal running and the vehicle is driven by two-wheel drive. When the front wheels which are the driving wheels slip, the speed of the vehicle drops and therefore, the rotational speed of the rear wheels drops. When the speeds of the inner and outer races of the clutch become equal to each other, the roller 30 bites into the cam surface 21a due to its wedge action and the clutch becomes connected so that the drive force is also transmitted to the rear wheels and a four-wheel drive condition is temporarily brought about. When thereafter the front wheels get out of the slipping condition by the four-wheel drive, the rotational speed of the inner race increases again relative to the outer race. Thus, the clutch again becomes disconnected and the vehicle is restored to the ordinary two-wheel drive condition. The abovedescribed action of the clutch during the backward movement is, of course, the same as that during the forward movement.
To make the two-way clutch operable in the directions of forward and backward movements as described above, it is necessary that the cage holding the roller be shifted by a predetermined amount in the circumferential direction during the changeover of the direction of movement of the vehicle. This shifting operation has heretofore been usually manually effected and has been accomplished by the driver shifting the position of the cage as by the lever operation when changing over the direction of movement. There is also known a method of shifting the position of the cage by the utilization of an outside actuator such as hydraulic pressure, but this requires a special energy source, which in turn leads to disadvantages such as high cost and bulkiness of the apparatus.